Analyzers have been provided for the detection of the concentration of liquid analytes using a analysis means, so-called dried test elements that contain within themselves the necessary reagents to permit such detection. Examples of such analyzers are described in U.S. Pat. Nos. 4,287,155, issued Sept. 1, 1981, and 4,340,390, issued July 20, 1982. Examples of such test elements appear in U.S. Pat. Nos. 3,992,158, issued Nov. 16, 1976; 4,053,381, issued Oct. 11, 1977; and 4,258,001, issued Mar. 24, 1981.
As is evident in the aforesaid U.S. Pat. No. 4,340,390, the preferred method of dispensing a small quantity of test liquid onto the test element is to transfer at least that quantity to be dispensed, from a first container into a temporary storage unit called a disposable tip, such as by aspiration, and then to pressurized such tip by amounts effective to dispense the small quantity of liquid onto the test element. To prevent loss of analyte and contamination, such first containers are covered by a cap designed to be pierced by the movement of the disposable tip into the container. The preferred material of the caps have been a plastic which permits teeth-like segments to be fragmented into bendable teeth during the piercing step. Because such teeth make the cap stick to the disposable tip, it has been necessary to provide means for stripping the cap and container off the disposable tip as the latter is withdrawn from the aspirating position within the liquid of the container.
Prior to this invention, such stripping means have been a fixed surface that encounters the cap after some movement of the tip has already occurred away from the aspirating position. As a result, stripping does not occur until the container has been lifted off its surface. I have discovered that this produces a final separation of cap and tip wherein the container and cap actually fall away from the tip under the influence of gravity, at a rate that is an order of magnitude greater than the nominal rate of movement of the tip.
A significant problem in the metering of liquid from such disposable tips has been that of perfusion. "Perfusion" is defined herein to mean movement of the liquid being dispensed, up the exterior surface of the tip rather than down onto the test element. As will be readily apparent, perfusion is totally unsatisfactory, as it renders unlikely that the desired amount, or indeed any amount, of liquid will be dispensed onto the test element. Considerable attempts have been made to reduce the tendency of dispensed liquid to perfuse. For example, the dispensing end of the disposable tip has been especially shaped and coated with a surfactant, as described for example in U.S. Pat. No. 4,347,875, issued on Sept. 7, 1982, to discourage the formation during aspiration of exterior liquid such as would be likely to encourage perfusion during subsequent dispensing. However, even such approaches, although very useful in reducing the number of perfusions, have not been totally successful.